The invention relates to aliphatic polycarbamates and to a process for their preparation.
Aliphatic polycarbamates, i.e. carbamates having more than one, preferably more than two, carbamates groups in the molecule, are known and are used inter alia as crosslinkers for the manufacture of automotive finishes. Since it is desired to lower the solvent content of coatings to reduce emissions, it is necessary, in order to retain a viscosity which is sufficiently low for application of the coating, to have available coating constituents of low viscosity.
EP-A 0 594 068 discloses a curable composition comprising a polycarbamate of the formula A 
where R1=H or CH3, R2=H, alkyl or cycloalkyl, L=a divalent linking group, A=a repeat unit derived from an ethylenically unsaturated monomer, x=10-90% by weight y=10-90% by weight and x+y=100% by weight.
It is an object of the present invention to provide novel aliphatic polycarbamates which, while being of low viscosity, contain two or more carbamate groups in the molecule, and whose molecular size and number of carbamate groups can be varied as desired, and also a process for their preparation.
We have found that this object is achieved by polycarbamates of the formula I 
in which
R and Rxe2x80x2 are identical or different and are alkyl groups having 1-4 carbon atoms and
n is a number from, on average, 2 to 5.
The invention further provides a process for the preparation of polycarbamates of the formula I, which consists in reacting a polyalcohol of the formula II 
wherein R, Rxe2x80x2 and n are as defined above, with phosgene, preferably in excess, to give a polychlorocarbonic ester of the formula III 
The polychlorocarbonic ester III is then converted to the polycarbamate I using ammonia.
Within the context of this description, polycarbamates and polyalcohols are understood as meaning those compounds whichxe2x80x94in any case on average in the case of homolog mixturesxe2x80x94contain more than 2 carbamate or OH groups, i.e. essentialy those representatives which can be referred to as oligomers. The polycarbamates according to the invention are frequently in the form of mixtures of homologs which primarily contain representatives where n=1 to approximately n=10.
The amination is generally carried out at a temperature of from 5 to 50xc2x0 C., preferably at 25 to 30xc2x0 C., and at a pressure of from 1 to 10 bar, preferably 1 to 3 bar. Here, it is possible to work in an inert solvent, such as toluene, chlorobenzene, heptane, dichloroethane and pentyl acetate. Preferably, the amination of the polychlorocarbonic esters is carried out without the addition of an inert solvent.
The compounds of the formula II are known and are described, for example, in DE-A 196 54 167. They are preferably obtained by catalyzed methathesis reaction of cyclopentene-containing hydrocarbon mixtures, for example petroleum fractions, hydroformylation of the reaction products and subsequent reaction with hydrogen over a hydrogenation catalyst.
The reaction of the polyalcohols with phosgene is generally carried out at a temperature of from +5 to 65xc2x0 C., particularly advantageously at a temperature of from +40xc2x0 C. to 45xc2x0 C. The phosgene is generally allowed to act in an excess of from 5 to 20 mol %, meaning that normally virtually all pi hydroxyl groups are converted to chloroformate groups.
The reaction of the polyalcohols can be carried out without the addition of inert solvents. However, it is generally advantageous to work in an aprotic solvent such as toluene.
The polycarbamates according to the invention are used as coating constituents, in particular in automotive finishes, where they are added to regulate the viscosity.
The examples below describe preferred embodiments of the invention. The OH numbers given therein signify g of OH per 100 g of substance. Determination of the OH number is described in DE-A 196 54 167.